Author Topic: Tiny Bubbles in my fuel pump bowl  (Read 1545 times)

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Offline GPS

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Tiny Bubbles in my fuel pump bowl
« on: Monday,April 28, 2025, 06:18:50 PM »
 So I took my 72 twin cam out around the hood this weekend and when I got home she showed me yet another puzzle. When I got home and shut her down I opened the the hood to check the engine for any leaks as I always do. I noticed small bubbles rising up in the glass bowl of my stock fuel pump. It looked like a glass of seltzer or an IPA. Open the gas tanks thinking there my be a vacuum somewhere but no changes. Started her back up again and got quite a bubble  show as the fuel bowl filled a little more. Never having seen the inside of a fuel pump, maybe this is normal??? Just want to not lay in bed at night and worry about it as I would normally do. I did remove the pump a few months back to clean up an oil leak from the block by replacing the gasket and resealing. In the process I took the pump apart and it all looked ok. The car seemed to run fine, so I don't think it is effecting it, but still curious. Does it need to be rebuilt?

Offline Grumblebuns

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Re: Tiny Bubbles in my fuel pump bowl
« Reply #1 on: Tuesday,April 29, 2025, 11:42:10 AM »
In all of the early years of TC ownership, I never noticed any air bubbles in the glass bowl. Possibly air is getting into the fuel lines under vacuum? Have you checked your fuel line connection to the fuel pump for tightness? It may not be worth rebuilding the mechanical fuel pump but instead order a replacement mechanical fuel pump. Unfortunately, I believe they do not come with the glass bowl. The other option that many of us have gone to in to install an electric fuel pump.


Offline 4129R

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Re: Tiny Bubbles in my fuel pump bowl
« Reply #2 on: Tuesday,April 29, 2025, 01:30:10 PM »
Those mechanical pumps are still available with glass bowls. SJS.

If there were any leak on the pipework, petrol would leak out, all over the hot exhaust and whoomph.

Been there, done that !!!

Offline Bainford

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Re: Tiny Bubbles in my fuel pump bowl
« Reply #3 on: Wednesday,April 30, 2025, 10:28:31 AM »
Grumblebuns may have your answer. I would check all fuel line connections upstream from the pump; at the tanks, the lower fuel 'T' (at the fuel tank cross-connect), at the fuel filter, and at the pump inlet itself. The inspection should include any threaded connections or adaptors at these points, as well. I would also inspect very carefully all fuel lines upstream of the pump for pin holes.

Is the issue more prevelent when the tanks are low on fuel? That would be another clue.
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Offline Kendo

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Re: Tiny Bubbles in my fuel pump bowl
« Reply #4 on: Wednesday,April 30, 2025, 11:15:19 AM »
If the pump is original and the diaphragm is starting to leak, would that produce bubbles?

Offline GPS

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Re: Tiny Bubbles in my fuel pump bowl
« Reply #5 on: Wednesday,April 30, 2025, 05:17:01 PM »
Thanks for all the insite.
The tanks are low as a matter of fact. I would say less then a quarter full.
I will check all the lines and connections. Thanks for that. Also has any one of you removed the scary nylon fuel line and replaced with hard line? Not a big fan of that much flexible line. It obviously works, I just don't like it. Installing hard line may be a bit of a trick however. Hmmmm.

Offline 4129R

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Re: Tiny Bubbles in my fuel pump bowl
« Reply #6 on: Thursday,May 01, 2025, 04:13:47 AM »
I have replaced all my fuel lines with 6mm copper tubing, with 6mm or 1/4" rubber hose connections.

The copper pipe  is used for gas supplies, and you can get brass compression T pieces, for the T joint where the 2 tanks feed the long pipe to the fuel pump.

Offline Bryan Boyle

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Re: Tiny Bubbles in my fuel pump bowl
« Reply #7 on: Thursday,May 01, 2025, 03:48:47 PM »
Thanks for all the insite.
The tanks are low as a matter of fact. I would say less then a quarter full.
I will check all the lines and connections. Thanks for that. Also has any one of you removed the scary nylon fuel line and replaced with hard line? Not a big fan of that much flexible line. It obviously works, I just don't like it. Installing hard line may be a bit of a trick however. Hmmmm.

If you install hardline, where it goes from a solid structure to something that could vibrate, you want to have some flexible line in there so you don't work harden/crack the solid line.  So, perchance, the cross-chassis intertank line may be hard, but the joints at the tanks themselves should be flexible.  If you do, then you should make sure the hard line itself doesn't vibrate by afixing it to the chassis (above the level of the floor so you don't catch it on something) instead of floating free, there are access holes in the chassis where the original line ran; grommet it, and run the line there. 

Flex hose from the "T" up to the pump, of course, as well as from the pump up to the carbs.  Going from the chassis to any vibrating part (like this)...flexible.

I've used the usual suspects' nylon line, with a wide bend up from the T to the pump, and it seems to be ok.  My head is always in the engine room, so, I check it regularly.  Maybe when I put the O/H tanks and new carbs in, I'll update to current fuel line. 
Bryan Boyle
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http://www.lotuseuropa.us for mirror of lotus-europa.com manual site.